Variable frequency oscillator



March 30, 1954 VARIABLE FREQUENCY OSCILLATOR Filed Oct. 31, 1946 7 Sheets-Sheet 1 FIG. 2A EB M C2 M T02 G GM ER, 1 i0"! %R3 1 2 E0 E0 E c FIG.38 I; FIG. 30 1 FIG. 30 j; i v

WOLCOTT M. SMITH INVENTOR ATTORNEY w. M. SMITH 2,673,932

March 30, 1954 w. M. SMITH. 2,

VARIABLE FREQUENCY OSCILLATOR Filed Oct. a1. 1946 7 Sheets-Sheet 2 Z0 Z0Z0 Z0 62 I G2 I G2 I R1 C8 $1 LA EB EB EB FIG. 4A FIG. 48 FIG. 4C

RGA Lc Z0 Z0 Z0 Z0 FIG.5A FIG. 5B F|G.5C FIG. 5D

Z0 Z0 Z0 Zq WOLCOTT M. SMITH INVENTOR ATTORNEY 7 Sheets-Sheet 3 R; S 1:E R45: :ERL R5 ig Re 03 1 r Eb C FIG. 9 F l G 4 G V1. G5 Cc 4 G G2 W. lGI C 2: R5 gRs I Ir ER I FIGJA- 6' IIOLCOTT a. 5mm 62 INVENTOR 61 Z g 7BY K ATTORNEY W. M. SMITH VARIABLE FREQUENCY OSCILLATOR March 30, 1954'7 Sheets-Sheet 5 Filed Oct. 31, 1946 Ec VOLTS Owmm m y/// w WOLCOi'T M.SMITH INVENTOR FROM G2 mom G4 ATTORNEY March 30, 1954 w, s n- 2,673,932

VARIABLE FREQUENCY OSCILLATOR Filed Oct. 31, 1946 7 Sheets-Sheet 7 Q)LEADING em 29 TYPE D LAGGING e61. Zg TYPE A 3800 L :100 uhy, y C =0 F0asas-m. V 802*: R3=25oo 3700 C2= 5o R 2=|5,ooo 4 3000 Fm F K.C.

Ec VOLTS FIG. I?

2 4 6 8 WOLCOTT M. SMITH INVENTOR A -Ec VQLTS FIG. 18 &5I x M ATTORNEYPatented Mar. 30, 1954 UNITED STATES PATENT OFFICE VARIABLE FREQUENCYOSCILLATOR Wolcott Marsh Smith signer, by mesne ass 18 Claims. 1

This invention relates to an oscillator circuit, the frequency ofoscillation of which may be varied instantaneously and over a relativelywide range in a simple manner.

This type of circuit is of particular value in frequency modulationtransmitters, in which the frequency of the transmitted signal must varyin accordance with and proportionately to the variations in frequency ofthe sound intelligence to be transmitted thereby. With the circuit ofthe present invention a variable frequency oscillator has been producedwhich is of exceedingly simplified and economical construction, whichemploys but a single vacuum tube, and which attains an exceptionallyhigh sensitivity of output l frequency to variations in applied voltage,the output amplitude retainin ahigh degree of amplitude stability. Thecircuit of this invention is susceptible of taking several forms, eachof which will achieve a specific objective either with regard tosensitivity or direction of response, range of frequency variation,constancy of output amplitude. In addition, the output resulting fromthis circuit has been found to be exceedingly rich in harmonics, so muchso that the amplitude of the lower harmonics is quite comparable to thatof the fundamental. As a result of this feature, the flexibility of useof the circuit is increased exceedingly.

This circuit is disclosed in a schematic or generalized way in Fig. 1 ofthe drawings appended hereto, Fig. 1 showing Zg connected to G2;

Figs. 1A and 1B are fragmentary reproductions of the schematic of Fig.1, but showing Zg connected respectively to the anode of tube V1 and tothe screen grids G3 and G;

Figs. 2A, 2B and 20 represent alternative embodiments of 276 of Fig. 1;

Figs. 3A, 3B, 3C and 3D represent alternative embodiments of Zg of Fig.1;

Figs. 4A, 4B, 4C and 4D represent alternative embodiments of Za of Fig.1

Figs. 5A, 5B, 5C and 5D represent alternative embodiments of Zc of Fig.1;

Figs. 6A, 6B, 6C and 6D represent alternative embodiments of Zo of Fig.1;

Figs. 7A, 7B, 7C and 7D represent alternative embodiments of Z1. of Fig.1;

Figs. 8A and 8B represent alternative embodiments of Zs of Fig. 1;

Figs. 9, 10, 11 and 12 represent various specific embodiments of thegeneralized circuit of Fig. 1, employing various combinations of theimpedances shown in Figs. 2 through 8;

Fig. 13 is a graph showing the variation in Springfield, Mass.,asignments, to General Instrument Corporation, Elizabeth, N. .L, acorporation 01' New Jersey Application October 31, 1946, Serial No.706,949

'ations in the frequency of E0.

peak amplitude of the output voltage of various circuits as the inputvoltage E0 is varied;

Fig. 14 illustrates the change in frequency of various circuits as theimput voltage E0 is varied;

Figs. 15, 16 and 17 are similar to Fig. 14, but relate to differentcircuits;

Fig. 18 is a graph showing the variation in R as E0 is varied forvarious values of R2.

The circuit of the present invention involves the use of but a singlevacuum tube V1, which is here illustrated as having tub elementsincluding a cathode 2, a plate 4, and a plurality of intermediateelectrodes comprising a tube control grid G1, oscillator elementscomprising an oscillator anode G2 and an oscillator control grid G4, aswell as screen grids G3 and G5, and suppressor grid Ge. Tubes having thestandard designations 7A8, 6A8 and 7B8 have been found to be exceedinglysuitable for use in this oircut, but other tubes which can perform thefunctions hereinafter to be described may be substituted in their place.A plurality of biasing circuits for the tube elements are provided,these including Z]: for the cathode, Za for the oscillator anode G2, Z0for the oscillator control grid G4, Zs for the screen grids G3 and G5,and Z1 for the tube anode 4. EB represents a positive biasing voltage sochosen in conjunction with the biasing circuits as to cause negativetransconductance to exist between the oscillator anode G2 and theoscillator control grid G4. A coupling circuit 20 is provided betweenthe oscillator anode G2 and the oscillator control grid G4, theimpedance Zc including a capacitor 01. E0 represents a suitable voltagesource which applies a voltage to the tube control grid G1, this voltagebeing variable by any convenient means between zero and negativequantities. The various impedances are so designed, as shall becomeclear hereafter, that the voltage E0, the output voltage of the circuit,has a frequency which is determined by and varies with the magnitude ofthe negative bias applied to G1 by Be. In discussing the operation ofthe circuit, Ec will generally be considered as having a definite value,

and it will be noted that for different values of E0, the frequency ofE0 will differ. Consequently, if a varying voltage is applied to E0voltage, the

at all times to the amplitude of a given sound. variations in E0determined by the frequency of the sound will be reflected incorresponding vari- By this means, the frequency of E0 will beproportional to the amplitude of the sound intelligence, andconsequently E0 will be frequency modulated by that sound.

The condenser C1 which forms a part of Z0, and more particularly thecharge and discharge rate of that condenser, determines the frequency ofthe output voltage E0. It accomplishes this result by modulating theelectron flow within the tube V1 between its cathode 2 and its plate 4,the frequency of this modulation being reflected in a correspondingfrequency change at E0.

As a further and very important feature, it has been found that if thetube control grid G1 be coupled to any one of the other tube elements,as by Zg, the effect of variations in E on the frequency of E0 isintensified and modified in various ways, depending upon the circuitdesign. The most marked effect attendant upon this coupling is a highlyincreased sensitivity of frequency response, that is to say, the curveof frequency against E0 becomes considerably steeper.

In Fig. 1, Zg is shown as connected to G2, but it may instead optionallybe connected to any of the other tube elements, different effects beingobserved in each case. Thus in Fig. 1A Zg is shown as connected to theanode 4 of the tube, in Fig. 1B Zg is shown as connected to the screengrids G3 and G5, and in Fig. 11 Zg is shown as connected to theoscillator control grid G4.

The symbols egl, Cg2, on and 6;) represent respectively theinstantaneous values of voltage on G1, G2, G1 and plate 4 respectively.

Figs. 2A, 2B and 2C illustrate three suggested circuits for providingthe cathode 2 with a bias, best operation of the circuit occurring whensuch bias is applied. All of these embodiments of Zlc are well-known tothe art, and it further will be apparent that if Zlc is omitted, aconstant negative bias applied to G1 will achieve the same end result.In all of the circuits shown in Figs. 9 through 12, a Zia correspondingto Fig. 23 has been employed, but this is merely a matter of choice.

Figs. 4A, 4B, 4C and 4D are illustrative of some of the various formswhich Za may take. The presence of Ca in Figs 4B and 4D modifies thewave shape of a e so that, as will be explained, Zg may cause a phaseshift to take place between cs1 and 3132. The circuits of Figs. 9 and 11employ the Za of 4A, while the circuit of Fig. 10 employs the of Fig.4B. The circuit of Fig. 12 employs a Za different from any of thoseillustrated in Fig. 4.

Figs. 6A, 6B, 5C and 61) represent various embodiments of Zo. Figs. 9,and 12 employ the of Fig. while Fig. 11 employs the Z0 of Fig. 6D. Thelatter embodiment defines a parallel resonant circuit having a naturalfrequency of oscillation. The presence of such a circuit either in Zo,as in Fig. 11, or in Za, as in Fig. 12, has a very marked effect on theoperation of the circuit, the current and voltage in the oscillatorycircuit assuming a substantially sinusoidal shape, thus facilitatingcontrolled phase shift by Zg, and the stiffness of the oscillatorycircuit moditying the frequency variations of the overall circuit.

Figs. 5A, 5B, 5C and 5D illustrate some of the various forms which Zcmay take. It will be noted that a condenser C1 is common to all of theseforms, this condenser servin to control the frequency of E0 by the timeit takes to charge and discharge. In Figs. 9 through 12, the Zc of Fig.5A is employed.

Figs. 7A, 7B, 7C and 7D represent some. of the various forms which Z1may take, this impedance serving not only to bias or apply high positivepotential to the tube anode 4, but also serv- 4 ing to couple the outputof the tube V1 to E0. The embodiments of Figs. 7A, 7B and 7C employ thecapacitive coupling of Ce, while the embodiment of Fig. 7D employsinductive coupling.

Figs. 8A and 8B are illustrative of various embodiments of Zs, thebiasing impedance for the screen grids G1 and G5. In Figs. 9 through 12,the embodiment of Fig. A is used throughout.

The various impedance designs illustrated in the above discussed figuresare but typical of circuit designs which may be employed in accordancewith this invention. Whichever of the specific embodiments is employedis in many cases a matter of mere choice and circuit design well withinthe ken of those skilled in the art. This is particularly true withrespect to the embodiments of Zlc, Zs and Z1. Certain other of thechoices will affect the operation of this circuit in manners hereafterto be describe.

The operation of the circuit without Zg and with G1 connected directlyto Fe will now be decribed. For purposes of illustration, thisdescription will be carried out with reference to Fig. 9, it beingassumed that R2 is infinite, and that therefore no coupling existsbetween G1 and G2, and it being further assumed that C1 and R3 areabsent from the circuit. The oscillatory portion of this circuit is madeup by Z0(Rg) Zc(C1) and Za(R1). It will be noted that this oscillatorycircuit is made up exclusively of resistive and capacitive elements,this particular circuit arrangement producing at E0 a succession ofsawtooth pulses. If C1. be omitted, these pulses will assume a squareshape. It will further be assumed that G1 is at ground potential.

Eb, via R1, applies a positive potential to G- and many of thoseelectrons which pass G1 are attracted to G2 and impinge thereon, thusdecreasing the positive potential of G2 and consequently decreasing theattraction it xerts on subsequent electrons. These electrons attractedto G2 pass through R1 thereby causing a voltage reduction at G2 which iscoupled to G1 through C1 causing G1 to become more negative, G4 thusdecreasing the flow of electrons through the tube to the plate 4, whichin turn decreases electron flow through R1. practically to zero so that6p approaches the value of EB. When G2 has accumulated so many electronsthat subsequent electrons are no longer attracted to it, the charging ofC1 will. cease, and electrons will leave G1 via Re, the condenser C1thus discharging. The two grids G2 and G1, and particularly G4, willtherefore each become more positive, and thus the electron flow throughV1 will increase and the current flow through R1. will increase, causinga corresponding decrease in the amplitude of e the variation. in e beingtransmitted via Cc to E0. When 2 has become positive enough to attractelectrons again, the voltage drop through R1 caused by the leakage ofelectrons from G2 will once again cause G1 to become more negative, andhence the cycle will repeat.

The rapidity with which these cycles occur will be determined by thevalues of Z0, Zc and Zn. and by the negative transconductance of thetube. The impedances are fixed by circuit design, but the value oftransconductance be varied with E0, a typical case being illustrated bythe lowermost curve of Fig. 18 labeled R2=oc. It will be noted from thatcurve that as Fe increasw. (becomes more negative), the value -R 5increases. R varies inversely as the transconductance between the gridsG2 and G4, and is experimentally determined by decreasing Rg untiloscillation just ceases. This variation in -R arises from the tubedesign, probably in the following manner: As Ec becomes more negative,fewer and fewer electrons are available for passage through V1. Thesmaller the number of electrons available, the less relatively negativemust G2 and G4 be charged in order to cut off electron flow through V1.Hence the fewer the electrons available the lower the potential to which01 must be charged in order to cut off electron flow. The lower thepotential to which C1 need be charged, the less time that charging willtake up, and the less will be its discharge time. The less the chargingand discharging time of C1, the more variations will take place in thepotential of G2 and G4 per unit time, and hence the greater will be thefrequency of those oscillations, thus giving rise to an increasedfrequency in E0. Thus a variation of E0 varies the transconductance ofthe tube which varies the charge and. discharge rate of the capacitor01, this in turn varying the frequency of output.

The circuit as thus far described therefore provides a simplearrangement in which the frequency of the voltage E0 may be variedmerely by changing the voltage Ec.

In order to modify or to intensify this effect, it has been founddesirable to impress a voltage c 2, eg, eg5, or 6;) on G1. This may beaccomplished by addition of impedance elements Zg to the circuit, thislatter impedance serving to couple G1 with one of the other tubeelements. confining our attention again to Fig. 9, and assuming that07:0, we find a Zg employed which corresponds to Fig. 3A. Figs 33, 3Cand 3D represent alternative embodiments for Zg. Y in these figuresrepresents a connection to any of the other tube elements. Z9 is hereshown as coupling G1 to G2. This coupling has the effect, in the circuitnow under discussion in which the oscillatory portion of the circuit iscomposed exclusively of resistive and capacitive elements, ofintensifying the effect of E0 on the frequency by what may be termedregenerative action. This is apparent from a study of the family ofcurves in Fig. 18. It is there seen that as the value of R2 decreases,thus increasing the coupling effect between the two grids, not only doesR increase for a given value of E0, thus leading to a higher frequency,but the slope of the curve markedly increases, thus giving rise to agreater change of frequency for a given variation in E0. This effectoccurs because in the circuit now under discussicn am will be in phasewith e 2, and hence gl will affect the frequency in a way similar to E0.Thus, comparing curves l and 4 of Fig. 14, curve i representing acircuit in which e 1 is taken from G2 and curve 4 representing a case inwhich e lzfl, it will be noted that for any given value of Fe thefrequency of curve I is higher than the frequency of curve 4.

If in this circuit Zg be connected to G4, a different result isobserved, such a connection giving rise to a reduction in frequency.Curve 2 of Fig. 14 illustrates this. It is believed that this effect maybe explained as follows: Because of the presence of C1, the voltage e ihas a wave shape different from e 2, oscilloscope observationsindicating that it contains a positive surge immediately following thenegative surge. These surges when applied to G1 apparently reduceelectron flow while G2 is attempting to acquire electrons, and toincrease electron flow when G2 no longer can use them. The reducedelectron flow increases the time necessary for C1 to charge G4 to thevalue necessary to cut down the later increased electron flow throughthe tube V1. Thus the frequency is decreased.

If Z9 is connected to the tube anode 4 (curve 3 of Fig. 14), the samefrequency-decreasing effect is observed as was the case when Zq wascoupled to G4, except that the magnitude of the effect is somewhatdecreased. This effect also is thought to be due to the variation inwave form between e and e z. Connecting Zg to the tube anode 4 has beenfound to have the additional effect of increasing the frequencystability of the circuit, making the oscillator as stable as any ood L-Cor inverse-feedback stabilized oscillator. It is notable that betteramplitude stability is I obtained with Zg coupled to the tube anode 4(curve 3 of Fig. 13). It will be noted that when egl is obtained from 6pthe output voltage E0 exhibits less variation than for the other curves,this amplitude stability being equal to that ob- 'tained with L-C andstabilized feedback oscillators, while still preservin an excellentratio of AF to ABC. The total output variation is but 40% in spite ofthe extreme range of frequency control, all of the curves of Fig. 14having a slope of approximately 8% Fc per volt of E0 where Fe is thecenter frequency of the linear portion of these curves. Prior systemsachieve a slope of approximately 2%, and the improvement which thiscircuit represents over prior circuits, even when Zg is omitted (curve4) will be apparent from this comparison.

Zg' may also be connected to G5, the frequency variation efiect thenapproximating that of curve 3 of Fig. 14. However, the wave shape of egSis apparently such that the amplitude stability achieved when 29 isconnected to the tube anode 4 is not achieved in this case.

In the circuit as thus far described, egl was effectively in phase withthe e 2 except when voltages having unusual Wave shapes were employed.It has been found that various desirable effects may be produced byforcing egl out of phase with e 2, either leading or lagging, theeffects being different when e 1 leads e z than when it lags. Thus ifthe Z0 of Fig. 6D be employed in the circuit, this impedance consistingof LG and CG connected in parallel, and if Za retain the form of Fig. 4A(Fig. 11), or if Z0 takes the form of Fig. 6A while Za contains aparallel resonant circuit (Fig. 12) these arrangements being typical ofmany which could be made in which the oscillatory portion of the circuithas a resonant frequency of vibration so that the current therein andtherefore the voltage assumes at least an approximately sinusoidal waveform, egl may be caused, by Zy, to differ in phase from e 2. The degreeof this phase difference may be controlled at will by design of Zg' in amanner well-known to the art.

When egl leads e z, the variations in electron flow past G2 precede itsdemand for electrons. Thus when G2 most wants electrons, the supply ofelectrons is not maximum, but is decreasing, while when G2 no longerwants electrons the flow of electrons is increasing. This tends to make20 take longer to become fully charged and discharged, since theelectrons available during the upswing of each cycle tend to preventthis change in G2, while the surge of electrons available during thedownswing of each cycle always occurs ahead of :the voltage r reductionat thereby tending to prevent reduction in e g. The final result ofthese trends islto force eachcycle to require a longer period,zthusresulting in a reduction of frequency.

When a lags g2 the effect is justthe opposite. The electrons availableduring'the upswing of each cycle are insuflicient and hence tend tocause e z to rise more rapidly, while the increase in availableelectrons during the downswing of each cycle tends to speedup thereduction of e 2. The final result of these trends is to force eachcycle to have ashorter duration, thus leading to an increase infrequency.

The action of Ecin the sinusoidal version of this oscillator now'underdiscussion apparently differs from its action when a pulse-typeoscillator in whichthe oscillatory circuit is composed exclusivelyofresistive-capacitive elements is employed. Variation ofEcalone withoutcoupling has been found to change the frequency of E only very slightly,this probably being due to the stiffness of the oscillatory circuititself. It is believed that the effect of E0 in this case may beexplained as follows: The voltage of G1 exercises primary control overthe number of electrons available for passage through V1. As the voltageof G1 becomes more negative, the number of electrons availabledecreases, and the speed of those electrons which pass G1 is alsodecreased. As E0 and therefore G1 become more negative, the effect whiche 1, thezcoupled voltage, has on the electron flow through the tube alsodecreases. In this way Ec controls frequency by control of the per centmodulation due to an of the electron stream passing G1.

Figs. 15, 16 andl'killustrate these effects. In Fig. 15, curverepresents the results obtained in a circuit in which Zg corresponds toFig. 3D (Fig. 12). C7, which may either represent a separate circuitelement 01' the inherent interelectrode capacitance of the tube V1between G1 and the cathode 2, further modifies egl over the effectthereon of the other elements of Z9. In this case,

it is noted that as E0 increases (becomes more negative), the frequencyincreases. When 29' takes the form of Fig. 3A (Fig. 11) egl is caused tolag g2 (seecurvefi) and hence as Ec increases (becomes more negative),the frequency decreases. Marked linearity of the curves is obvious.

The circuit :constants for the curves of Fig. were so chosen thatoscillation took place at relatively low rates (Fo=l26 k0,). Withdifferent circuit constants curves (Fo=3525 kc.), these curvesevidencing a very high frequency sensitivity to variation in E0. Fig. 17compares curves 9 and I0, somewhat different circuit constants beingemployed.

All of these curves show a marked linearity over relatively widevariations in E0, and show exceedingly high frequency sensitivities withregard to varations in Be.

It will be apparent that in accordance with the considerations abovediscussed, various specific circuits may be formed from the genericcircuit of Fig. 1. 'By'way of exempliiication only, and not by way oflimitation, four such circuits are illustrated in Figs. 9 through 12.The circuit of Fig. 9 is of the.resistive-capacitive, orpulsating type.In this circuit, Zk takes the form of Fig. 2B, Zg takes the form of Fig.3B, Zc takes the form of Fig. 4A,'Zc.takes the formof Fig. 5A, Z0 takesthe form'of'Fig. 6A, Zs takesthe form of Fig. 8B, and 1Z1. takes theform of Fig. -'7A. 01.

1 and 8 were'developed resonant circuit, composed elsewhere illustratedinthe drawings.

of vibration. When 'Zg known in the art. When causes the pulsesot E0tot-assume asawtooth form, and if C1. is omittedthose pulseswill assumea square shape. C3 and Cfcre' bypass condensers.

The circuit of Fig.1!) differs from that of Fig. 9 in omitting C1. andin adding Ca so that Z0 takes the form of Fig. 4B. Thepresence of C8causes thevoltage applied to R2 to have a modified wave form such thatZy can impart a .phase shift thereto, and therefore by suitabledesigning Zg any desired phase shift, and consequently any desiredfrequency-grid bias relationship may be obtained.

Figs. 11 and 12 are representative of circuit designs in which theoscillatorypart of the circuit has a resonant frequency. Fig. 11 showsthe ofLcand Co, as part of Z0, thus corresponding to Fig. 6D. Fig. 11also shows .Gi coupled to the oscillator control grid G4, the couplingimpedance Zg taking the form of Fig. 3C. Fig. 12 has the resonantcircuit portion consisting of L-C as part of Za, it being added to theZa of Fig. 4B to define a Za not A comparison of Figs.'11 and 12indicates that-whether the resonant portion of the circuit forms part ofZ0 or Za, essentially the same effects are produced, theseeffects'differing'in degree and/or sign only, and not in kind.

From the above description, the true flexibility and adaptability of thecircuit of the present invention W131 be apparent. By combining impeances into the arrangement of Fig. 1, the application of a varyingvoltage'Ec to the tube control grid G1 of tube V1, which is biased to acondition of negative transconductance, will so modulate the electronflow through the tube as to cause it to vary at a frequency which itselfvaries to follow the changes in the voltage Be. This effect obtains evenif Zg -is absent, the-presence of .Zg intensifying the'effect. It is' tobe noted that this effect obtains,- even though no part of the circuitmay besaid to have a resonant frequency is present, it may couple G1 toany one of the other: electrodes of the tube, and when that couplingobtains the output frequency may becontrolled by varying either theamplitude wave form, or phase shift of the coupled voltage egl, hesechangesbeing obtained by choice of proper circuit design in a mannerwellthe oscillatory portion of the circuit has a resonantfrequency, the.output frequency may still be varied by changing the voltage E0.

The circuit here described is of exceedingly high sensitivity, ascompared with previous circuits accomplish'the same end, thissensitivity being achievedswithout loss of high outputamplitude'stability. Th circuit employs but a single vacuum tube, isrelatively simple, and requires no critical balancing orzhigh precisionof circuit-componentsiinorderfor it to work. It

has'been.observedthattthe output from this circuit is exceedingly rich.in harmonics, thus permultiplication .by the employand trap circuits inmitting' frequency ment of suitable resonantthe output.This.last;.mentioned factor materially extends the field ofapplicability of this circuit.

'It will be apparent that many variations may be'made in the :specificcircuitdesign, without departing from thespirit of the invention as setforth in the appended claims.

I- claim: 1. A variable frequency oscillator comprising avacuumtubehavingas tubeelements a cath ode, a plate, and at least threeintermediate electrodes there-between constituting, in order fromcathode to plate, a tube control grid, an oscillator anode, and anoscillator control grid, biasing circuits connected to said tubeelements for maintaining said oscillator anode at a higher potentialthan said tube control grid and said cathode, said plate at a hi herpotential than any of the other tube elements, and said tube controlgrid at a potential sufficiently below that of said cathode so thatnegative transconductance exists between oscillator anode and oscillatorcontrol grid, a coupling circuit including a capacitor connected betweenthe oscillator anode and oscillator control grid, a second couplingcircuit substantially non-frequency-discriminating over the frequencyrange of operation of the oscillator and including a capacitor connectedbetween the tube control grid and one of the tube elements other thanthe cathode and any tube element connected directly to the cathode, andadditional means for independently varying the voltage applied to thetube control grid and thereby varying the frequency of os llation and ofsaid oscillator.

2. The iable frequency oscillator of claim 1. in which the couplingcircuit to the tube control grid in conjunction with the biasingcircuits for the oscillator anode and the oscillator control grid definea circuit which causes the voltage coupled to the tube control grid todiffer in phase from the voltage of the oscillator anode.

3. The variable frequency o illator of claim 1 in which said secondcoupling uit is connected between the tube control grid and theoscillator anode.

4. The variable frequency oscillator of claim 1 in which said secondcoupling circuit is connected between the tube control grid and saidoscillator control grid.

5. The variable frequency oscillator oi claim 1 in which said secondcoupling circuit is connected between the tube control grid and theplate.

6. In the variable frequency oscillator of claim 1, a pair of connectedtube elements defining a screen grid on either side of said oscillatorcontrol grid, a biasing circuit connected to said screen grid formaintaining it at a potential appropriate for functioning as a screengrid, said second coupling circuit being connected between the tubecontrol grid and said screen grid.

7. The variable frequency oscillator of claim 1, in which the couplingcircuit to the tube control grid in conjunction with the biasingcircuits for the oscillator anode and the oscillator control grid definea circuit which is composed exclusively of resistive and capacitiveelements.

8. The variable frequency oscillator of claim 1, in which the couplingcircuit to the tube control grid in conjunction with the biasingcircuits for the oscillator anode and the oscillator control gridtogether include an oscillatory circuit having a resonant frequency ofvibration.

9. The variable frequency oscillator of claim 1, in which the biasingcircuit for one of the oscillator elements comprises inductive andcapacitive elements in parallel.

10, The variable frequency oscillator of claim 1, in which theoscillator circuit is so designed as to impart a substantiallysinusoidal shape to the voltage on the oscillator elements and in whichthe tube control grid is coupled to one of the oscillator elements.

11. The method of varying the frequency of oscillation of the output ofa vacuum tube having as tube elements a cathode. a. plate, and aplurality 10 of intermediate electrodes including a tube control gridand oscillator elements including an oscillator anode and an oscillatorcontrol grid, said method comprising causing negative transconductanceto exist between the oscillator anode and oscillator control grid,maintaining an oscillatory condition between said oscillator anode andoscillator control grid, varying the voltage applied to the tube controlgrid, and applying to said tube control grid substantially all of thefrequency components, within the frequency range of the operation of theoscillator, of the varying voltage of one of said tube elements otherthan the cathode.

12. The method of varying the frequency of oscillation of the output ofa. vacuum tube having as tube elements a cathode, a plate, and aplurality of intermediate electrodes including a tube control grid andoscillator elements including an oscillator anode and an oscillatorcontrol grid, said method comprising causing negative transconductanceto exist between the oscillator anode and oscillator control grid,maintaining an oscillatory condition between said oscillator anode andoscillator control grid, varying the voltage applied to the tube controlgrid, and applying to said tube control grid the varying voltage of saidoscillator anode.

13. The method of varying the frequency of oscillation of the output ofa vacuum tube having as tube elements a cathode, a plate, and aplurality of intermediate electrodes including a tube control grid andoscillator elements including an oscillator anode and an oscillatorcontrol grid, said method comprising causing negative transconductanceto exist between the oscillator anode and oscillator control grid,maintaining an oscillatory condition between said oscillator anode andoscillator control grid, varying the voltage applied to the tube controlgrid, and applying to said tube control grid substantially all of thefrequency components, within the frequency rwge of the operation of theoscillator, of the varying voltage of said oscillator control grid.

14. The method of varying the frequency of oscillation of the output ofa vacuum tube having as tube elements a cathode, a plate, and aplurality of intermediate electrodes including a tube control grid andoscillator elements including an oscillator anode and an oscillatorcontrol grid, said method comprising causing negative transconductanceto exist between the oscillator anode and oscillator control grid,maintaining an oscillatory condition between said oscillator anode andoscillator control grid, varying the voltage applied to the tube controlgrid, and applying to said tube control grid substanially all of thefrequency components, within the frequency range of the operation of theoscillator, of the varying voltage of said plate.

15. The method of varying the frequency of oscillation of the output ofa vacuum tube having as tube elements a cathode, a plate, and aplurality of intermediate electrodes including a tube control grid andoscillator elements including an oscillator anode and an oscillatorcontrol grid, said method comprising causing negative transconductanceto exist between the oscillator anode and oscillator control grid,maintaining an oscillatory condition between said oscillator anode andoscillator control grid, varying the voltage applied to the tube controlgrid and applying to said tube control grid a varying voltagecorresponding to that of one of said tube elements other than thecathode and any tube element connected directly to the cathode butdiflering in 11 phase therefrom, said varying voltage includingsubstantially all of the frequency components, within the frequencyrange of operation of the oscillator, of said tube element.

16. The method of varying the frequency of oscillation of the output ofa vacuum tube having as tube elements a cathode, a plate, and aplurality of intermediate electrodes including a tube control grid andoscillator elements including an oscillator anode and an oscillatorcontrol grid, said method comprising causing negative transconductanceto exist between the oscillator anode and oscillator control grid,maintaining an oscillatory condition between said oscillator anode andoscillator control grid, varying the voltage applied to the tube controlgrid, and applying to said tube control grid a varying voltagecorresponding to that of the oscillator anode but differing in phasetherefrom.

17. The method of varying the frequency of oscillation of the output ofa vacuum tube having as tube elements a cathode, a plate, and aplurality of intermediate electrodes including a tube control grid, ascreen grid, and oscillator elements including an oscillator anode andan oscillator control grid, said method comprising causing negativetransconductance to exist between the oscillator anode and theoscillator control grid, maintaining an oscillatory condition betweensaid oscillator anode and oscillator control grid, varying the voltageapplied to the tube control grid, and applying to said tube control grida varying voltage corresponding to that of said screen grid.

18. A variable frequency oscillator comprising a vacuum tube having astube elements a cathode,

,a plate, and at least three intermediate electrodes therebetweenconstituting, in order from cathode to plate, a tube control grid, anoscillator anode, and an oscillator control grid, biasing circuitsconnected to said tube elements for maintaining said oscillator anode ata higher potential than said tube control grid and said cathode, saidplate at a higher potential than any of the other tube elements, andsaid tube control grid at a potential sumciently below that of saidcathode so that negative transconductance exists between oscillatoranode and oscillator control grid, a coupling circuit including acapacitor connected between the oscillator anode and oscillator controlgrid, a coupling circuit including a capacitor connected between thecontrol grid and the oscillator anode, and additional means forindependently varying the voltage applied to the tube control grid andthereby varying the frequency of oscillation of said oscillator.

WOLCOTT MARSH SMITH.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,180,369 Philpott Nov. 21, 1939 2,226,561 Herold Dec. 31,1940 2,393,717 Speaker Jan. 29, 1946 2,396,088 Crosby Mar. 5, 19462,412,782 Palmer Dec. 17, 1946 FOREIGN PATENTS Number Country Date563,421 Great Britain Mar. 8, 1944

